Basically TV cameras includes a pick-up tube that is a transducer which converts optical images (spatial variations in brightness) into electrical signals (temporal variations in current). The signals are amplified and processed by the camera circuitry to provide video signals that can be displayed by a monitor, recorded on a magnetic tape or transmitted by a broadcast TV transmitter, for example. The invention is concerned with methods of improving the operation of the camera tube.
Camera tubes have three basic functions:
a. photo-detection i.e. converting light images into electrical charges used to discharge the storage on the dielectric target of camera tubes; PA1 b. charge storage i.e., charging the dielectric target; and PA1 c. signal read-out; i.e., reading out the dischaged portions of the dielectric target. PA1 charging the target with an electron beam that scans a field areas of said target during a field scan period, said electron beam normally focused to encompass a single elemental area PA1 blanking said electron beam during a discharge period, PA1 exposing said target elemental areas to photons during said discharge period, thereby discharging certain of said target elemental areas, PA1 recharging said target elemental areas during a read period to generate a signal current further recharging said target elemental areas with said electron bean during a scrub period and defocusing said electron beam to encompass a plurality of said target elemental areas during said scrub period.
Signal read out is accomplished by using an electron beam which scans the target in a video manner to analyze the changes in the electrical charges occuring between successive beam scans. The electron beam charges the target and thus each scan restores the target to a charged state wherein the beam side of the target is at the tubes cathode potential (Vc) and the window side of the target is at another potential (Vw) so that there is a potential difference (Vwc) across the target dielectric. Photons striking the target discharge the target, decreasing the potential difference across the target. When the electron beam strikes discharged sections of the target, electrons flow from the beam to the target inducing current to flow from the potential (Vc) through a load resistor to the window side of the target. This current flowing through the resistor is the output current i.e. the output signal of the TV tube.
Thus the optical image is placed on the camera by the photons which discharge target elements. The analog amount of the discharge is detected by the electron beam sweeping the target elements to recharge the elements. Ideally as the illumination (brightness) changes the discharge of the target element should change linearly and proportionally. However, there is a time lag between the change in the illumination and the change in TV tube output signal. This time lag occurs both when decreasing and when increasing the target illumination and is known as "lag". In the light intensity decreasing phase it is known as "decay lag" and in the light intensity increasing phase it is known as "build-up lag".
In many operations, the TV tube lag is insignificant to the output of the tube; however, in medical imaging operations and especially in medical imaging using DSA (i.e. Digital Substration Angiography) the lag causes significant problems. The decay lag causes instantaneous data for constructing real time images to be mixed with the data of prior acquired images; i.e., preent images include the data of the previous images. The build up lag degrades the value of the first few acquired images to the point where they cannot be used in the DSA technique. The lag problem is aggravated in DSA because of the light pulsing mode used in the DSA operations.
Present DSA systems attempt to overcome the lag problem by either using light biasing or by increasing the number of target scrubs. It has been found that the lag decreases as the light bias increases and as the number of scrubs increases. It has further been found that all of these methods are cumulative.
The lag is effected by the storage capacitance of the target element; which is in effect a photoconductor, and by the resistance of the scanning beam at low signal level. The decay of the signal is affected by the RC time constant of these equivalent capacitances and resistances. For practical purposes there is no video signal (neglecting dark current) when the photo conductor face is fully charged since in that condition there is no beam electron deposition onto the photoconductor face.
It is desirable in DSA techniques that all acquired images of the non-time dependent objects are as identical to each other as possible. The lag or the RC decay effects cause the image data during the first few images to vary considerably. This characteristic is a problem unique to the DSA acquisition technique. In the DSA technique, before the first X-ray pulse, the electron beam scans the target for a relatively long time period (i.e. the time period necessary, for example, to accomplish a plurality of frame scans). The pre X-ray multiple scans are done in darkness and result in fully charging the photocoductor face; i.e. the target is saturated. The saturation occurs since the cumulative time used for the pre X-ray scan is much greater than the RC time constant of the TV tube; that is, the time constant due to the equivalent resistance-capacitance of the target.
The X-ray pulse is applied after saturation. The X-ray energy is transformed into visible light pulses and results in the generation of a number of electron hole pairs. The electron hole pairs discharge the photoconductor face of the target. During the discharge time the electron beam is blanked.
Subsequently, a progressive scan is used to read out the information electrostatically stored. The inherent RC characteristic causes lag to occur in the charging of the dielectric faces. This lag as previously noted varies the information of the first few frames and thus makes them impossible to use. Similarly, the data obtained by the scanning beam is erroneous due to the lag, since this data includes prior acquired data along with the newly acquired data.
In the prior art, attempts have been made to correct for the errors occurring when the image data and residual image data are combined and due to build-up degradation. These attempts comprise either using light biasing arrangements or increasing the number of scrubs of the target photo- conductor face. Lag decreases as the light bias increases and as the number of scrubs increase. Both procedures, i.e. scrubbing and biasing, require extra time or equipment. A well done scrubbing, in particular, requires many scrub periods and results in a long image time interval which is unacceptable in usual DSA procedures. Therefore new and improved methods and systems of overcoming the lag problem are desired, especially for use in DSA systems.